There is a demand for fluorinating agents which are site-selective towards organic, especially carbanionic, substrates, especially for use in the preparation of pharmacologically active compounds. A number of such electrophilic fluorinating agents are known but are limited to their commercial utility in that they are expensive, hazardous, inconvenient to handle, lack stability and/or are insufficiently selective for general use.
Fluorine (F.sub.2) solutions in halogenated or other suitable solvents at low temperature (e.g. -78.degree. C.), trifluoromethyl hypofluorite (CF.sub.3 OF), cesium fluoroxysulfate (CsSO.sub.4 F) and perchloryl fluoride (FClO.sub.3) are all active electrophilic fluorinating agents (S. T. Purrington et al, Chem. Rev. 1986, 86, 997 and G. G. Furin in "New Fluorinating Agents in Organic Synthesis", Ed. L. German and S. Zemskov, Springer-Verlag: Berlin, 1989, 35-68) but are either not sufficiently selective or too hazardous for general use. Xenon difluoride (XeF.sub.2) is potentially less hazardous but is too expensive in many applications.
Recently attentiaon has been directed to the use of compounds of the N-F class, i.e. having an N-F bond, as electrophilic fluorinating agents. The prototypical member of this class is perfluoro-N-fluoropiperidine (R. E. Banks and G. E. Williamson, Chem. Ind. (London), 1964, 1864 and R. E. Banks et al, J. Chem. Soc., Perkin Trans. I, 1972, 1098). However, this compound is obtainable only in low yields by electrochemical fluorination of pyridine (about 8% yield) or 2-fluoropyridine (about 13% yield) in anhydrous hydrogen fluoride. Furthermore, it has been found to be inadequately reactive in several applications and, on transfer of fluorine to a carbanionic substrate, liberates the imidoyl fluoride perfluoro-1-azacyclohex-1-ene which then competes for the substrate. Similar problems militate against use of the analogous compounds perfluoro-(N-fluoro-2,6-dimethylpiperidine) and perfluoro-N-fluoromorpholine (R. E. Banks et al, J. Chem. Soc., Perkin Trans. I, 1988, 2805) and of poly[perfluoro-(N-fluoropiperidin-4-ylethylene)] (R. E. Banks & E. Tsiliopoulos, J. Fluorine Chem., 1986, 34, 281) as electrophilic fluorinating agents.
Other members of the N-F class include N-fluoropyridin-2(1H)-one (S. T. Purrington and W. A. Jones, J. Org. Chem. 1983, 43, 761 and J. Fluorine Chem., 1984, 26, 43); N-fluoro-sulfonamides (U.S. Pat. Nos. 4,479,901, 4,828,764 and DE 3623184 A); camphor-derived enantioselective N-fluoro-sultams (E. Differding and R. W. Lang, Tetrahedron Lett., 1988, 29, 6087); N-fluoroquinuclidinium salts (R. E. Banks et al, J. Chem. Soc. Perkin Trans. I, 1988, 2805 and R. E. Banks & I. Sharif, J. Fluorine Chem., 1988 41, 297), N-fluoro-N-alkyl-2,2,6,6,-tetramethyl piperidinium chlorate (I. V. Vigalok et al, J. Org. Chem. USSR, 1983, 19, 1203), and N-fluoropyridinium salts (Umemoto et al, Tetrahedron Lett. 1986, 27, 3271 & 4465 and T. Umemoto & G. Tomizawa, Bull. Chem. Soc. Jpn., 1986, 59, 3625). N-Fluoroquinuclidinium fluoride and triflate and N-fluoro-N-methyl, -propyl or -neopentyl -p-toluene-sulfonamides are available commercially.
U.S. Pat. No. 4,828,764 discloses that certain N-fluoro-N-perfluoroalkyl or perfluoroaryl sulfonamides including, inter alia, those of the formula R.sub.f SO.sub.2 NFR are electrophilic fluorinating agents. In said formula R.sub.f represents a perfluorinated C.sub.1 -C.sub.30 alkyl, C.sub.3 -C.sub.30 cycloalkyl, C.sub.6 -C.sub.14 aryl substituted C.sub.1 -C.sub.10 alkyl or a C.sub.6 -C.sub.14 aryl group and R represents a C.sub.1 -C.sub.30 alkyl, C.sub.3 -C.sub.30 cycloalkyl, C.sub.6 -C.sub.14 aryl substituted C.sub.1 -C.sub.10 alkyl, or C.sub.6 -C.sub.14 aryl group optionally substituted with one or more inert substituents including, inter alia, fluorine and, when R.sub.f is trifluoromethyl, R alternatively can represent perfluoromethylsulfonamido. The preferred fluorinating agents are stated to be N-fluorobis(trifluoromethanesulfon)imide (R.sub.f =CF.sub.3 and R=CF.sub.3 SO.sub.2) and N-fluoro-N-methyl-trifluoromethanesulfonamide (R.sub.f =CF.sub.3 and R=CH.sub.3). The former compound (also known ad DesMarteau Reagent) is a powerful electrophilic fluorinating agent which is capable of fluorinating benzene to fluorobenzene at room temperature, but is tedious to prepare requiring eight or nine reaction steps from readily available material.